1. Field of the Invention
The invention relates to a screw extruder mechanism, in particular a twin-screw extruder, for the processing of strongly outgassing materials.
2. Background Art
In the processing of materials of this type as it is used for instance for the degassing of volatile components from viscous fluids such as polymer melts, polymer solutions, pastes etc., high quantities of gas or steam develop, the discharge of which poses some problems. In addition to the fact that for instance a certain residual solvent content is reached, a restriction of these processes is imposed by the operationally reliable discharge of the high quantities of steam or gas that develop. This is true in particular for fluids the content of which in terms of volatile components exceeds 10% --in the extreme as much as 90% in polymer solutions,--for moist powders or in case water is fed into the extruder as an auxiliary agent for the final degassing.
The maximum possible quantity of steam to be discharged operationally reliably in a degassing process is determined by the structure of the process part of the extruder as well as by its way of operation. For instance, the number, position and geometry of the degassing ports as well as the configuration of the screws are substantial design criteria.
Screw extruder mechanisms are known from DE 24 00 271 C2 and DE 32 48 659 A1, conventionally comprising a main extruder for proper material processing, a material supply device in the form of a feed hopper, a material discharge device in the form of a discharge die, possibly a feed device for a degassing agent to be fed into a portion of evaporation of the main extruder (as in the case of the mechanism according to DE 24 00 271 C2), and a degassing device in the main extruder for the gas stream to be carried away from the portion of evaporation. Solvents, emulsions, moist bulk materials, pastes or the like can be supplied to the extruder by way of the feed device.
In the extruder mechanism according to DE 24 00 271 C2, degassing takes place by countercurrent flow degassing, steam being blown into a closed zone of the extruder, removing solvents from the material to be processed. The mixture of water vapor and solvent vapor is discharged through a vent in the form of an ancillary twin-screw extruder that opens radially into the main extruder. The screws of the ancillary twin-screw extruder which form a material barrier have such a pitch and are driven in such a direction that the solids taken along during degassing are pressed back to the material to be treated in the main extruder.
In areas that are separated from the above vent by means of pressure locks, provision is made for further degassing ports of conventional design.
The degassing activity achievable by the extruder specified above is limited, use being made regularly of only one degassing device at a time in a certain portion of evaporation of the extruder.
In the screw extruder according to DE 32 48 659 A1, provision is likewise made for two degassing devices which are mounted in varying areas of the extruder and each of which may consist of twin-screw extruders. The two degassing devices are each in connection with an independent decompression zone, a damming zone working as a separation between the two decompression zones. Consequently, degassing takes place in two isolated separate streams.
In the cases described, the flow of vapor or gas is discharged at a position upstream and/or downstream of the respective place of evaporation seen in the axial direction. For domination of the quantities of gas developing, the ports may vary in size and shape, the maximum quantities being limited nevertheless.
Furthermore, the operational reliability of a degassing port decisively depends on the gas velocity in the screw spirals and on the outlet edge of the screw channel towards the degassing port. The gas velocity again depends on the pressure (over- or underpressure) applied to the degassing port. In any case, it is a fact that the gas velocity depends on the volume rate of the gas or steam developing in the portion of evaporation so that during the processing of strongly outgassing materials and whenever major quantities of steam develop, the gas velocity will rise to critical ranges which can be controlled no longer or only with difficulty by conventional techniques.